Are Habitable Exoplanets Really Habitable?

Every few months, the press describes some recently discovered exoplanet as the closest thing yet to Earth's twin. But how much like Earth are these planets really? And do we even have the ability to answer that question?

Lots of headlines have touted the discovery of potentially habitable worlds, some of which might be Earth twins. Now, typically these stories are accompanied by pictures that look a lot like planets from science fiction movies, with a solid surface, liquid water, and a surface gravity that humans could at least function in. But unfortunately, those are all artist renditions. These exoplanets don't actually look like that. Or maybe they do. No one knows. In fact, we have no idea whether any of these planets have water or oxygen, let alone whether you could just walk out of a ship and hang out there. It's not that the press or astronomers are lying. There's just a major disconnect between what many people imagine when they hear the phrase "habitable world," and what astronomers mean by that same phrase.

To astronomers, the phrase "habitable exoplanet" just means exoplanet that lies in the habitable zone of its host star. And the habitable zone, in turn, is defined as the sweet spot of orbital distances from that star at which the energy from starlight would produce the right temperature on a planet's surface for water to remain liquid, provided the planet has a surface and provided there's enough atmospheric pressure. Now, the bad news is that being in the habitable zone isn't even remotely close to a guarantee of actual human habitability. For instance, Venus has a solid surface and it has plenty of atmospheric pressure. Plus, if you expand its orbit just a little bit, it would be in the sun's habitable zone. But there's no liquid water there. So is the habitable zone, instead, a prerequisite for habitability? Not exactly. A planet with a super-thick atmosphere, for example, could have surface water in a larger orbit than you'd ordinarily expect. The habitable zone is more of a guideline, a starting point to narrow down targets of interest. Those estimates you hear of an average of one habitable planet per star in the Milky Way are really just statements about this starting point.

To make a more definitive assessment of habitability, actual habitability, for any exoplanets, you have to analyze their atmospheres. But we don't know anything about the atmospheres of any of the so-called Earth-like habitable worlds that get reported in the press. In fact, we can't be certain that they even have atmospheres at all. It could be Earth-like, or it could be barely there, like on Mars, or a carbon dioxide super-greenhouse like on Venus, or it there could be no atmosphere at all. We just don't know.

What about habitable zone exoplanets that are closer to us? Could we measure properties of their atmospheres and locate a truly habitable world like that? Well, you can measure the atmosphere of an exoplanet, but not if that exoplanet is an Earth-sized rocky body in a star's habitable zone. Our two methods for determining a planet's atmosphere only work for planets that are in very large orbits outside the habitable zone, or really close to their stars. So you see the problem. To be in the habitable zone, a planet has to be small enough to be rocky like Earth instead of gaseous like Jupiter. And it can't be too close or too far from its star, or it won't have liquid water. But unless it is really close or really far, its atmosphere cannot be measured easily, and you can't know whether it really might be habitable. It's a catch-22, but only because of the limitations of current instruments.

So does that mean identifying the only potentially habitable worlds is a waste of time? No. On the contrary, it's critical. We have to narrow the field. And we need to improve our census of what kinds of exoplanets are out there. It's important science. Unfortunately, we don't have the means to identify Earth 2.0 yet.